Transfer Cases for Doubler - 71 GMC Jimmy

[RckyMntnKng]

Ok this thing needs a doubler, and Ive had a hard time finding a good used deal on a np205 with the 32 spline from a th400. That's what everybody calls for...ORD, NWF etc.

My truck has/had a Th350 with the 27 spline np205 stock, and so the bearing is smaller and to change the input shaft would be a pain or more expensive than waiting to find the right transfer case.

While back I found a Th350 style Np203 and adaptor from a 74 or so blazer, so I think i can mount the range box to my Th350....

Well i found a very cheap 205 from an 86 chevrolet 1-ton that was mated to a TH400. Looks to have the 32 spline long input shaft (would have to be changed for short shaft, but bearing size is correct?), and I guess a slip yoke rear output (Not sure if I keep that part, original 205 didnt have that style). It also has some sort of PTO that ran a 12k generator. All this stuff works flawlessly and the date on the 205 is March 13th 1986. I was told all 86 and newer 205's were a round style instead of racetrack 8 style, but this one appears to have the same 8 bolt pattern to me, unless they are talking about the tranny side of the adaptor

So now I have all kinds of crap, any advice on what I need or don't need to make the doubler?

What is the best and most affordable doubler kit manufacturer?

Anybody know anything about this style or brand of PTO?
Would a PTO winch work well if I wanted to try that route?

Any tips, pointers, or help is appreciated!

First 3 Pics are the 86 np205, 4th pic is the 203 and adaptor, 5th is the stock 205 and 27 spline adaptor

[ghettoluxury]

the 203 will not bolt up to your tranny the tailshafts are not the same length between 203 and 205

[duallyjams]

Try researching here http://offroaddesign.com/catalog.htm

[OrrieG]

I think there was one on Boise Craigslist a couple of weeks ago for $200.

OOPs didn't look at your photos, looks like you bought it, I remember the PTO setup.

[RckyMntnKng]

the 203 will not bolt up to your tranny the tailshafts are not the same length between 203 and 205

-Actually it does bolt right up as I pulled it directly from another 27 spline th350 tranny.

I found the info on ORD's page:

The TH350 transmission is very common and was coupled to the NP203 in MANY GM trucks. The TH350 transmission will need a 27 spline output shaft that sticks out of the case by about 7/8". This output configuration was used with NP203 and NP205 transfer cases.

I think there was one on Boise Craigslist a couple of weeks ago for $200.

Yes I got that one, and for less than $200. It was from a M1031, 1&1/4 ton military chevrolet with the pto used for a 12k generator. Its a nice transfer case...just would like a SYE kit and the short 32 spline input to make it work. Maybe I can re-use the standard yoke from my original 205, but havnt tried that yet.

ORD says the doubler kit uses the just the range box portion of the 203 mounted to the th350, then the adaptor kit to add the 205 behind that,....giving a possible 4:1 low range. You can twin or triple stick the setup.

[ghettoluxury]

I guess you found the info you needed and yes you should be able to pull your yoke to replace the one in the new case

[OrrieG]

I think ORD make adapter so you can "clock" (rotate) the 203 unit so it does not hang so low. Would the pto work with a winch??

[2tons of fun]

yes it will, but pto winches bring major hassel in operating.

your 32spl 205 does apear to have the long input, it shouldnt, mine didnt, but yes the bearing is correct.

the 203 will bolt up to the th350 with zero issues. its the th400 that has different length shafts depending on wether it had a 203 or 205.

the rear slip yoke can be swapped out for the fixed yoke from the 27spl 205. be carefull when dissasembeling them, there is a set of loose roller bearing inside the rear shaft and they will fall out and into the guts of the case.


however, after all of this, the reality is that your doubled th350 is still not as much reduc as a stock 465/205.

[vtblazer]

Originally Posted by 2tons of fun

however, after all of this, the reality is that your doubled th350 is still not as much reduc as a stock 465/205.

Not true when you throw in the torque converter (x2):

SM465 x 205 combo - 1st gear/low range = 12.84
Th-350 x 203 x 205 combo - 1st gear/low range/low range = 19.85

Times whatever axle gears you have will give you your final crawl ratio.

[RckyMntnKng]

Yes, I'm shooting for the 100:1 range.

This truck will be more of a toy for hitting some mud and gnarly camping spots, and not necessarily a hard core crawler made soley for Rubicon or Fordyce. Although, with the incoming 19.85 x 4.11 axle gearing I should get a first run with about 82:1 ratio. That should let me hit the highways when needed. If I find it doesnt have enough umph or is to low in rpm at 65-70 mph, I can easily go 4.56 / 4.88 and perfect the highway power range and help further my crawl with 90 or 97:1. I think this will be an all around performer without being limited to just one aspect. If I find myself someday at the entrance to the rubicon....Im sure I can have alot of fun with 80:1 or lower and a winch and lockers

[vtblazer]

100:1 is considered by many to be the optimum gearing for crawling.

4.88's will get ya to 97:1

5:13's would be 101:1


What size tire are you going to be running?

What type?

[RckyMntnKng]

Thanks for the info

Right now Im gonna run and finish out a set of used 39" boggers until I work all the kinks out, then on the next set I will research into what works best.

Ive read alot about all the different tires, so will see what terrain i get into the most

[2tons of fun]

Originally Posted by vtblazer

Not true when you throw in the torque converter (x2):

SM465 x 205 combo - 1st gear/low range = 12.84
Th-350 x 203 x 205 combo - 1st gear/low range/low range = 19.85

Times whatever axle gears you have will give you your final crawl ratio.

6.55 x 1.96 = 12.84

2.48 x 2 x 1.96 = 9.72

what people say about the t/q is bs. ive wheeled both, i will never wheel a auto again. it is nowhere near the same.

all in all it will cost you about 1800-2500 to setup a doubler.

what would it cost you to swap in a 465 which is insanely stronger and a smarter choice???

[-Greg72]

There's a lot more to the discussion than simply multiplying a couple of numbers together....

The fact is that the stall effect of a TC will affect your overall gear ratio and your crawl ratio, the question is how well you are set up to take advantage of it with your stall speed and engine combination.

If the stall speed is not well-matched to the power of the engine you will need a lot of throttle to generate the wheel torque needed to climb the obstacle. If you have to mash the throttle to get the power you need, you won't be able to take advantage of the extra "gearing" of a slipping TC. Once the converter stalls you are correct...the ratio is 1:1 and there is no extra gearing advantage over the 2.48 first gear ratio.

The Doubler is a time-tested and proven solution for better crawl ratio... and when it was released there was no other way to get REALLY deep gearing. Today there are numerous options in a single transfer case that will get you deeper than 4:1 reduction. Clearly, the 465/205 combo is not the final word in deep gearing.

[2tons of fun]

you are right, a 465/205 isnt the final word in it.









but my 465 doubled is, its insane. if i put it in front wheel drive i can slide the rear tires and i can dead drag about 14,000lbs

[vtblazer]

Originally Posted by 2tons of fun

but my 465 doubled is, its insane. if i put it in front wheel drive i can slide the rear tires and i can dead drag about 14,000lbs

A doubled 465 gets down into the "useless" catagory, a gear that's cool to say you have but in reality, does you no good.

As for skidding the front tires, Rcky's set up will do that too without breaking a sweat.

You running twin or triple sticks?

[2tons of fun]

b.s. that low gearing is useless, it makes wheels hard rocky trails a breeze. i got the setup installed and used it the next day to pull 30 some odd trees out of the ground clearing property.

becuase of my stupid steep driveway i use it to push my project 2ton all the time. granny gear will stall out, throw it double down and it will gently and easily creep it right where i want it.

i have mine 3x stick. it only takes about 1hr to 2x stick the 205 and it is worth every minute.